Potable water is a precious resource, yet it is one that is increasingly under threat from a torrent of polluters. Amongst the countless man-made contaminants that infiltrate our water sources are heavy metals. Usually as byproducts of industrial processes, if ingested in even trace amounts, these materials pose many serious health risks to humans, risks that include damage to internal organs, the central nervous system and the reproductive system, as well as side effects such as nausea and vomiting.
In the last three decades, in response to a growing awareness of the hazards presented by pollutants in the water supply, governments have enacted legislation to control discharges of waste. In particular, major acts, such as the Clean Water Act, identified heavy metals as substances requiring aggressive regulation. Consequently, industries, ranging from metal mining, manufacturers of computers and other electronic components, producers of fertilizers, to power generation facilities, have sought various means to remove metal ions from their waste streams before they reach natural bodies of water.
Amongst the methods of heavy metal extraction currently practiced are precipitation (often using electrochemical cells), reverse osmosis, use of paramagnetic nanoparticles, biological degradation by specially engineered bacteria, and ion exchange. The last of these, ion exchange, is particularly attractive to producers of large volumes of waste, especially those in the power generation industry which produces vast quantities of water contaminated with heavy metals every day.
Ion exchange is a separation process that has found profitable application in separation of closely similar metal ions. The underlying principles of ion exchange technology, and examples of typical resins, are familiar to one of ordinary skill in the art (see, e.g., Principles and Practice of Analytical Chemistry, F. W. Fifield, and D. Kealey, International Textbook Company, (1983), at pages 130-138). In brief, ion exchange apparatuses comprise an insoluble stationary phase—usually a porous resin—attached to which are fixed charge-carrying groups. Mobile counter ions of opposite charge reversibly exchange with solute ions in a mobile phase that travels across the resin. Variations in reversible exchange rate give rise to differential mobilities.
Accordingly, ion exchange has been applied to waste streams from a number of industrial processes. For example, ion exchange is widely used for polishing operations to reduce residual heavy metal and other pollutants to very low levels in order to meet National Pollutant Discharge Elimination System (NPDES) permit requirements or to satisfy the stringent quality thresholds required for re-use of waste products. Solid phase methods based on ion-exchange resins have provided very convenient application and recovery of the extractant and are particularly appropriate for removal of heavy metal contaminants from non-nuclear power plant effluents, where ready regeneration of the saturated resin is desirable and where introduction of toxic organic solvents into the environment must be avoided. Examples of chelating agents that have been deployed for heavy metal sequestration include dithiocarbamates.
There are two principal advantages of ion exchange processes. One is that quality effluent is attainable; a second is that specific species can be targeted for removal. However, a major disadvantage of current ion exchange technology, apart from that inherent in any batch process, is the relatively large volume of acidic wastes and flush waters that are needed. The attendant hazards of handling concentrated acids (and bases) have also been recognized.
Despite those disadvantages of current systems, ion exchange remains a technology of interest. Important characteristics of an ideal wastewater treatment resin would include: 1) high affinity for the target metal ions, which may be present in wastewater at relatively low concentrations (e.g., <100 ppm); 2) relatively low affinity for other metal cations, to avoid premature inactivation of the resin that would lead to increased regeneration cycles; and 3) variable metal affinity in response to some easily changed system parameter, such as the pH. Commonly, simple cation exchange resins exhibit deficiencies in one or more of these areas. For example, benzenesulfonate resins have relatively low heavy metal affinities and selectivities and require strong acids to release other bound metal ions when they are regenerated. Needless to say, the concentrated mineral acid required for the regeneration process poses operator safety, corrosion and disposal issues.
In some types of ion exchange processes, complexing agents with chelating functional groups that have selective affinities for certain metal ions are attached to the resin. With suitable choice of chelating group, target metal ions can be slowed sufficiently in their passage across the resin that they are effectively sequestered. Thus, the use of organic complexing agents for the selective removal and recovery of metal ions from aqueous solutions is a proven technique, and both solid support and immiscible liquid extraction have been utilized (see, e.g., Rydberg, J.; Musikas, C.; Chippin, R. G., Principles and Practices of Solvent Extraction, New York, (1992)).
Before considering whether a complexing agent is suitable for use with a resin, it is typical to consider its properties in solution. The usual categories of compounds currently used as extractants for heavy metal ions in various liquid-liquid extraction methods are: 1) α-hydroxyoximes; 2) phosphorus-bonded oxygen-donor compounds; and 3) acidic organophosphorous compounds (see, e.g., Kakoi, T.; Ura, T.; Kasaini, H.; Goto, M.; Nakashio, F., “Separation of Cobalt and Nickel by Liquid Surfactant Membranes Containing a Synthesized Cationic Surfactant”, Separation Science and Technology, 33, 1163-1180, (1998); Elkot, A. M., “Solvent Extraction of Neodymium, Europium and Thulium by Di-(2-ethylhexyl)phosphoric acid”, J. Radioanalytical and Nuclear Chemistry-Articles, 170, 207-214, (1993); Mathur, J. N.; Murali, M. S.; Krishna, M. V. B.; Iyer, R. H.; Chinis, R. R., et al., “Solutions of Purex Origin using Tributyl-phosphate”, Separation Science and Technology, 31, 2045-2063, (1996)).
The oxime, or hydroxy-imino, function strongly binds metal ions, particularly transition metal ions. This function has been used primarily in liquid-liquid extractions of metals, with extractant molecules that incorporate both a hydroxy group and the oxime to enable bidentate chelation.
Neutral organophosphorous esters have demonstrated the ligating power of the neutral phosphonate group, which is due to its high polarity. For example, the tri-n-butyl phosphate group is highly polar, having a dipole moment of 3.0 Debye units and a relatively high dielectric constant (8.0), and has been extensively used as an extractant for actinides and lanthanides (see, e.g., De, A. K.; Khopkar, S. M.; and Chalmers, R. A., Solvent Extraction of Metals, p. 259, Van Nostrand Reinhold Company, New York, (1970)). Neutral organophosphorous esters solvate electrically neutral metal-anion ion pairs, formed by suppression of their ionization in aqueous solution, and, therefore, function satisfactorily only in the presence of a highly concentrated salting-out electrolyte. The high extractive power of these reagents has been demonstrated for a large number of metal salts, typically nitrates and chlorides (see, e.g., Marcus, Y.; Kertes, A. S., Ion Exchange and Solvent Extraction of Metal Complexes; (p. 1037 of the 1970 edition); Wiley Interscience, New York, 1969). However, neutral organophosphorous esters have had no direct relevance to heavy metal abatement in industrial effluents hitherto principally because the phosphonate group has very little chelating power.
Simple acidic organophosphorous reagents extract metals in aqueous solution essentially by a cation exchange reaction between the replaceable proton of a phosphonic acid OH group and the coordinating metal cation. In the majority of extraction processes that utilize these reagents, the phosphonic acid RP(O)(OH)2 group entering into the exchange reaction is only singly-ionized, i.e., one of the protons remains unexchanged. In organic solvents, dialkyl phosphoric monoacids are usually dimers, and the resulting metal chelates are generally represented as M(HA2). Typically, these reagents have been used in liquid-liquid extractions and thus incorporate long lipophilic ‘tails’: e.g., monododecyl-phosphonic acid, used for extraction of U(VI) or Fe(III), and mono-n-butyl-, monoisobutyl- and monoisoamyl-phosphonic acids, used for extraction of protactinium (see Bodsworth, C., The Extraction and Refining of Metals, CRC Press, London, (1994)).
Given the success of these organic ligands with single functional groups as chelating agents for heavy metal ions, attempts have been made to incorporate two or more groups into a single ligand. As is well understood, bidentate ligands offer significant thermodynamic advantages over mono-dentate ligands, a property referred to as the “chelate effect” (see, e.g., F. A. Cotton, and G. Wilkinson, Advanced Inorganic Chemistry, (4th ed., Wiley, 1980), at page 71). Principally, there is an entropic benefit from taking half as many bidentate ligands out of solution into a complex as monodentate ligands would be taken. Additionally, of course, fewer molar equivalents of a bidentate ligand are required to achieve the same chelating effect as for a monodentate ligand.
β-hydroxyoximes are highly selective metal complexing reagents that preferentially chelate ions of nickel, molybdenum, copper and certain other transition metal ions. The oxime group increases the acidity of the neighboring alcohol group, thereby enhancing bidentate ligation. The extraction equilibrium can be represented by equation (1):M2+(aq)+2RH(org)=R2M(org)+2H+(aq)  (1)
Equation (1) shows that the OH protons on the ligand (denoted RH) exchange with the metal ions, the equilibrium position being governed by the overall hydrogen ion concentration. Structure 1 is a typical β-hydroxyoxime reagent that has been used to extract metal ions from acid solutions. Exemplary alkyl substituents, denoted R, include C9H19 and C12H25.

Oxime and phosphonate groups can be combined into a single molecule to form a free bidentate ligand for metals (see, e.g., Breuer, E., Acylphosphonates and Their Derivatives: The Chemistry of Organophosphorous Compounds, p. 685, John Wiley & Sons, New York, (1996)). In general, the simple α-(hydroxyimino)phosphonic acids and their monoesters have been made as E isomers only, see Breuer, E., Acylphosphonates and Their Derivatives: The Chemistry of Organophosphorous Compounds, p. 685, John Wiley & Sons, New York, (1996). Examples in which the ligand coordinates to the metal in a bidentate chelating mode through the oxime nitrogen atom and a phosphonate (P═O) oxygen atom, include: the diester, diethyl (E)-α-hydroxyimino-p-methoxybenzylphosphonate, which forms isolable complexes with Co, Ni and Cu dications; and the E isomer of monoester monoacid phosphonate versions of these complexes that contains one available POH group and one POR ester group (where R is an alkyl group, for example, ethyl). Formation constants and metal binding selectivities have not been reported for these ligands.
Phosphonocarboxylates have been reported to have enhanced complexation properties. Phosphonoacetic acid (PAA), which has found limited use as an extraction agent for some lanthanide series elements, was found to ligate a range of metal dications (see, e.g., Farmer, M. F.; Heubel, P.-H. C.; Popov, A. I., “Complexation Properties of Phosphonocarboxylic Acids in Aqueous Solutions”, J. Solution Chemistry, 10, 523-532, (1981); and Stunzi, H.; Perrin, D. D. J., Inorg. Biochem., 10, 309-318, (1979)). Complexation with such ligands involves intramolecular coordination by both the phosphonate and the carboxylate groups. Cu2+ is especially tightly bound by such ligands, with a Kf (equilibrium complex formation constant) of 108, but alkaline earth dications are less well bound, having Kf values of around 102-103. Thus, these species have high discriminating power for various cations. Transition metals are preferentially bound by the trianionic form of the ligand prevalent at pH >˜6-7. The related phosphonocarboxylate, phosphonoformic acid, complexes transition metals about as well as pyrophosphate at slightly alkaline pH, despite the higher negative charge of pyrophosphate under such conditions, thus confirming the superior complexing power of the phosphonocarboxylate ligand (see, Song, B.; Chen, D.; Bastian, M.; Martin, B. R.; Sigel, H., “Metal-Ion-Coordinating Properties of a Viral Inhibitor, a Pyrophosphate Analogue, and a Herbicide Metabolite, a Glycinate Analogue”, Helvet. Chim. Acta, 77, 1738-1756, (1994)).
The combination of neighboring oxy-imino and carboxyl groups in a single ligand can also lead to markedly enhanced chelating ability. Thus, 2-cyano-2-(hydroxyimino) acetic acid, 2-cyano-2-(hydroxyimino)acetamide and 2-(hydroxyimino) propanohydroxamic acid have been found to be powerful ligands for both Cu2+ and Ni2+ (see, e.g., Sliva, T. Y.; Duda, A. M.; Glowiak, T.; Fritsky, I. O.; Amirhanov, V. M., et al., “Coordination Ability of Amino-Acid Oximes—Potentiometric, Spectroscopic and Structural Studies of Complexes of 2-Cyano-2-(hydroxyimino)acetamide”, J. Chem. Soc. Dalton Trans., 273-276, (1997); and Sliva, T. Y.; Dobosz, A.; Jerzykiewicz, L.; Karaczyn, A.; Moreeuw, A. M., et al., “Copper(II) and Nickel(II) Complexes with Some Oxime Analogs of Amino Acids—Potentiometric, Spectroscopic and X-ray Studies of Complexes with 2-Cyano-2-(hydroxyimino)acetic acid and its Ethane-1,2-diamine Derivative”, J. Chem. Soc., Dalton Trans., 1863-1867, (1998)).
Recently, it has been recognized that α-(hydroxyimino)phosphonoacetic acids (also called phosphonoglyoxylic acid oximes, “α,α-disubstituted trifunctional oximes”, or “Troika acids”) are useful as pH-sensitive chelating agents. See, e.g., U.S. Pat. No. 5,948,931 to McKenna and Kashemirov, incorporated herein by reference in its entirety. Troika acids are molecules in which all of three potential metal coordinating groups—phosphonate, oxime and carboxylate moieties—are anchored to a common (α) carbon atom. Thus, Troika acids have three powerful functional groups that can coordinate heavy metal ions: a phosphonic acid group, P(═O)(OH)2 (phosphonate when ionized); an oxime group, ═N—OH; and a carboxylic acid group, C(═O)(OH) (carboxylate when ionized); all of which are attached to an anchoring central carbon atom and each of which is ionizable according to ambient pH (see, Kashemirov, B. A.; Ju, J.-Y.; Bau, R.; McKenna, C. E., “‘Troika Acids’: Synthesis, Structure and Fragmentation Pathways of Novel α-(Hydroxyimino)phosphonoacetic acids”, J. Am. Chem. Soc., 117, 7285-7286, (1995)). The three groups, phosphonic acid, oxime and carboxylic acid, are depicted from left to right in each of structures 2a and 2b.
An important feature of these compounds is that they have a tri-fold functionality, hence the name Troika.
Troika acids have unique properties not found in other chelating agents used in the art. For example, the mode of chelation for the Troika acids is different from common chelating agents such as ethylenediaminetetraacetic acid (EDTA). Specifically, a ligand such as EDTA coordinates a metal ion directly through an amine nitrogen atom, whereas a Troika acid coordinates through an oxime nitrogen atom.
Additionally, by virtue of its unique central location in the Troika acid structure, the oxime OH group can hydrogen-bond with either of its two neighboring groups, giving rise to two isomeric configurations, (E or Z), according to the particular conditions (see, e.g., Kashemirov, et al., J. Am. Chem. Soc., 117, 7285-7286, (1995)), as illustrated in structures 2a and 2b. The two isomers are designated “E” and “Z” based on the orientation of the N—OH in space. Each of the two isomers has different properties. Thus, the oxime hydroxyl group significantly influences, if not directs, the chemical reactivity of either of its two neighboring groups, depending upon its position.
Furthermore, not only are Troika acids capable of strong metal complexation under specific conditions, but they can be designed to release the chelated cations through changes in condition, such as pH. However, if Troika acids are to find application in ion exchange, and, in particular to the sequestration of heavy metal ions found in effluents such as those from power-plants, ways must be found to incorporate them into the stationary phases of ion exchange apparatuses.
The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known, or part of the common general knowledge at the priority date of any of the claims.
In addition, throughout the description and claims of the specification, use of the word “comprise” and variations thereof, such as “comprising” and “comprises”, is not intended to exclude other additives, components, or steps.